Meter for the measurement of multiphase fluids and wet gas

ABSTRACT

A meter for the measurement of the gas mass flow rate and total liquid mass flow rate of a two phase or multiphase fluid is disclosed. The meter comprises a series of three flow element, the firs element is arranged to mix fluids passing therethrough and the second and third elements are each arranged to provide a measurement of the differential pressure across that element. The second and third elements may be added to an existing installation with an element capable of performing mixing, after the mixing element. The second and third elements provide independent measurements by having different configurations presented to the fluids passing therethrough. The first and third elements may comprise conduits with internal projections and the second element may be a venturi. A flow element for use in such a meter is also disclosed.

[0001] Hydrocarbon fluids produced from wells consist of a mixture ofgaseous hydrocarbons, liquid hydrocarbons and water. It is desirable tometer these hydrocarbon fuels in their so-called two-phase state wherethe term two phase is taken to mean that the fluid consists of both agas phase and a liquid phase. The liquid phase can be made up of two ormore constituents, typically oil and water. Where the volume of gas ishigh with respect to the volume of liquid the multiphase well fluid isoften referred to as wet gas.

[0002] Methods and apparatuses for measuring multiphase flow are known.For example WO 99/56091 discloses a method of measuring a gas massfraction in a mass of liquid and gas flowing along a pipeline. Themethod includes providing a flow conditioner to create a uniform mixtureof gas and liquid and a venturi positioned downstream from the flowconditioner. By measuring differential pressures at points across and/orbetween the two flow elements the gas mass fraction of the liquid andgas flow may be determined.

[0003] According to a first aspect of the present invention there isprovided a meter for measurement of multiphase fluids comprising aseries of at least three flow elements. The first of the three elementsis preferably arranged to mix fluids passing therethrough and the secondand third elements are preferably differential pressure based flowmeasuring devices. The second and third elements may be provided afteran existing first element which can perform a sufficient degree ofmixing. The differential pressure based flow measuring devices arepreferably configured to provide measurements that are independent ofeach other. Each of the flow elements may create some form of pressuredrop along its length. The first and third elements preferably have aninternal geometry which may be formed by one or more internalprojections and the second element preferably includes a venturi.

[0004] Preferred examples of the first aspect of the present inventionhave been demonstrated to provide highly accurate and consistentmeasurements of both gas and liquid flow rates using measurements ofpressure or differential pressure. The accurate and consistentmeasurements are maintained even at medium, high and very high gas voidfractions.

[0005] According to a second aspect of the present invention there isprovided a flow element comprising a conduit having at least twoprojections therein. The flow element preferably has two projectionswhich project inwardly from substantially opposite inner surfaces of theflow element. The projections are preferably staggered along an axiallength of the conduit. Such a flow element thoroughly mixes andhomogenises mixtures of fluids passing therethrough. A differentialpressure may be measured across the flow element.

[0006] Each aspect of the invention will now be further described by wayof example, with reference to the accompanying drawings, in which:

[0007]FIG. 1 diagrammatically shows a meter for measurement of two phaseor multiphase fluids and

[0008]FIG. 2 shows a slot element which can be used as one of the flowelements.

[0009] An example of a flow-measuring device illustrating this inventionis shown in FIG. 1. This device consists of three substantiallycylindrical flow elements (1, 2, 3) connected in series in this case bymeans of flanges (5, 6). Within the first element (1) are two wedges (8,9) projecting toward the bore axis. Each wedge (8, 9) has a firstsurface (10, 13) oriented obliquely to the bore and generally facing theupstream end of the meter (31), a second surface (11, 14) that runsparallel with the bore axis and a third surface (12, 15) orientedobliquely to the bore and generally facing the downstream end of themeter (32). A given wedge (8, 9) has a substantially semi-cylindricalperipheral surface, the curved portion of which conforms to thecylindrical inner wall of the bore. In this example the two wedges (8,9) are staggered and located such that the upstream wedge (8) is at thetop of the cylindrical bore, and the downstream wedge (9) is at thebottom of the bore. There is a gap between the upstream surface (13) ofthe downstream wedge (9) and the downstream surface (12) of the upstreamwedge (8) to enable the wedges to protrude by more than 50% of the crosssection of the bore without totally inhibiting the flow.

[0010] The second element (2) is a well-known venturi. This element issubstantially cylindrical and consists of a convergent section (16), a‘throat’ section (17) and a divergent section (18). A pressure tapping(19) upstream of the convergent section (16) and a further pressuretapping (20) in the throat section (17) allow a differential pressuremeasurement (DP1) to be made.

[0011] The third element of the meter (3) is similar in design to thefirst element (1) and includes two wedges (21, 22). Each wedge (21, 22)has a first surface (23, 26) oriented obliquely to the bore andgenerally facing the upstream end of the meter (31), a second surface(24, 27) that runs parallel with the bore axis and a third surface (25,28) oriented obliquely to the bore and generally facing the downstreamend of the meter (32). A given wedge (21, 22) has a substantiallysemi-cylindrical peripheral surface, the curved portion of whichconforms to the cylindrical inner wall of the bore. In this example thetwo wedges (21, 22) are staggered and located such that the upstreamwedge (21) is at the top of the cylindrical bore, and the downstreamwedge (22) is at the bottom of the bore. There is a gap between theupstream surface (26) of the downstream wedge (22) and the downstreamsurface (25) of the upstream wedge (21) to enable the wedges to protrudeby more than 50% of the cross section of the bore without totallyinhibiting the flow. Element three (3) differs from element one (1) inthat two tappings are made in the body (29, 30), one tapping (29) beforethe upstream wedge (21) and one tapping (30) after the downstream wedge(22). These tappings (29, 30) facilitate the measurement of thedifferential pressure (DP2) across element three (3) of the meter.

[0012] In the present example all the tappings (19, 20, 29, 30) arelocated at substantially the uppermost position and are taken verticallyupwards to the pressure measuring devices (DP1, DP2, 34). The meter,consisting of the three flow elements (1, 2, 3), is connected in to apipe line usually by means of flanges (4, 7) with the flow directionflowing from the upstream end (31) to the downstream end (32). The metershould preferentially be orientated in the horizontal position. Ameasurement of the upstream pressure (34) should also be taken alongwith a reading of the fluid temperature (35).

[0013] The differential pressure, upstream pressure and temperaturemeasurements (DP1, DP2, 34, 35) are fed back to a processing element(33) that contains the algorithms to compute the gas and liquid flowsand also possibly an annunciation of flow rates and process conditions.These algorithms are described later. Alternatively the processingelement (33) can use look-up tables to determine the gas and liquidflows from the measurements.

[0014] The role of flow element one (1) is one of mixing. In multiphaseflow at high gas volume fractions it is usual for the gas to travelalong the centre of the pipe at a much higher velocity than the liquid,which tends to adhere to the wall.

[0015] Alternatively, it is also common for the majority of the liquidto flow along the bottom of the pipe at a much lower velocity than thegas above it. The difference between the in-situ velocities is oftentermed slip. The wedges (8, 9) of flow element one (1) cause the liquidto be drawn off the wall or from the bottom of the pipe and into the gasflow. This creates a mixing that is key to the performance of the secondand third flow elements two and three (2, 3) of the meter.

[0016] The second and third flow elements (2, 3) of the meter aredifferential pressure based flow measuring devices. It is important thatthe configuration of these two differential pressure elements is suchthat independent measurements are provided. This means that the elementsmust behave differently in the presence of liquid.

[0017] The gas and liquid flow rates may be derived in the followingmanner:

[0018] First the mass flow rate is measured from the second and thirdelements under the assumption of single phase, dry gas conditions.$\begin{matrix}{{Q_{ma} = {\frac{C_{a}}{\sqrt{1 - \beta_{a}^{4}}}ɛ\frac{\pi}{4}d_{a}^{2}\sqrt{2\quad \Delta \quad P_{a}\rho_{g}}}},} \\{Q_{mb} = {\frac{C_{b}}{\sqrt{1 - \beta_{b}^{4}}}ɛ\frac{\pi}{4}d_{b}^{2}{\sqrt{2\quad \Delta \quad P_{b}\rho_{g}}.}}}\end{matrix}$

[0019] C is the discharge coefficient, d the effective restrictiondiameter, ΔP the differential pressure, ρ the gas density, β the ratioof d to the pipe diameter and the subscripts a and b denote element twoand three respectively.

[0020] The presence of liquid in the fluid stream causes the measuredflow rates, Q_(ma) and Q_(mb), to be larger than the true gas mass flowrate, Q_(gc). This overread can be related to the Martinelli parameter Xas, ${X = {\frac{1 - x}{x}\sqrt{\frac{\rho_{g}}{\rho_{l}}}}},$

[0021] where,

[0022] x is the gas mass fraction, often termed quality when related tosteam,

[0023] ρ is the density, where the subscripts g and l denote gas andliquid respectively.

[0024] The relationship between overread and Martinelli parameter may beexpressed,${\frac{Q_{m}}{Q_{gc}} - 1} = {{M\frac{\left( {1 - x} \right)}{x}\sqrt{\frac{\rho_{g}}{\rho_{l}}}} + {c.}}$

[0025] Provided there is sufficient demarcation between the two DPelements (2, 3) with respect to the constant M then two independentequations relating Q_(m) and x to Q_(gc) exist,$Q_{gc} = {\frac{Q_{ma}}{1 + c_{a} + {M_{a}\frac{\left( {1 - x} \right)}{x}\sqrt{\frac{\rho_{g}}{\rho_{l}}}}} = {\frac{Q_{mb}}{1 + c_{b} + {M_{b}\frac{\left( {1 - x} \right)}{x}\sqrt{\frac{\rho_{g}}{\rho_{l}}}}}.}}$

[0026] Solving for the gas mass fraction or quality, x, yields,$x = \frac{r_{\rho}\left( {r_{Q}M_{a -}M_{b}} \right)}{\left( {1 + c_{b} - {M_{b}r_{\rho}}} \right) - {r_{Q}\left( {1 + c_{a} - {M_{a}r_{\rho}}} \right)}}$

[0027] where $r_{Q} = \frac{Q_{mb}}{Q_{ma}}$

[0028] ratio of measured flows under the assumption of single phase gasflow. $r_{\rho} = \sqrt{\frac{\rho_{g}}{\rho_{l}}}$

[0029] square root of the ratio of the density of gas to liquid.

[0030] Once the quality or gas mass fraction has been ascertained thenthe gas mass flow rate may be calculated,$Q_{gc} = {\frac{Q_{ma}}{1 + c_{a} + {M_{a}\frac{\left( {1 - x} \right)}{x}\sqrt{\frac{\rho_{g}}{\rho_{l}}}}}.}$

[0031] Finally with a knowledge of the gas mass flow rate and the gasmass fraction the liquid mass flow rate is calculated,$Q_{l} = {\frac{Q_{gc}\left( {1 - x} \right)}{x}.}$

[0032] The geometry of the upstream mixing flow element (1) and thethird flow element (3) are key factors in providing an accuratemeasurement of both liquid and gas. The first element (1) upstream mixeris particularly important in that it maintains a predictablerelationship between the Martinelli parameter and the particular meterelement overread for both DP elements (2, 3). The unique internalgeometry of the third element (3) ensures that this relationship issignificantly different from the second element (2) to allowsatisfactory resolution of the gas mass fraction equation.

[0033] The first element, thoroughly mixes and homogenises the flow. Theexample shown in FIG. 1 shows the first element comprising a speciallydeveloped internal double wedge geometry. However, any flow mixer orflow homogeniser will be suitable as the first element such as a mixerplate for example. The second and third elements each have adifferential pressure measurement taken across them. The second andthird elements behave differently in the presence of fluid to produce asubstantially different relationship to liquid flow from each other sothat independent differential pressure measurements are provided. Thisis achieved by the second and third flow elements presenting differentconfigurations to the fluids flowing therethrough. The example of FIG. 1shows a venturi as the second element and a double wedge arrangement asthe third element. However any two elements which present differentconfigurations to the fluids flowing therethrough so that independentdifferential pressure measurements are obtained will be suitable. Forexample as well as the venturi and double wedge mentioned above, a slotelement such as the example shown in FIG. 2 could be used instead ofeither the venturi or double wedge. The slot is machined into in a solidbar (100) of material of similar diameter to the meter spool. Themachined slot (101) runs axially through the bar (100) having asymmetrical cross section in the shape of a rectangle or a letter O withconcentric curved faces top and bottom and parallel vertical faces oneach side. Furthermore the two differential pressure measuring elementsmay be in any order.

[0034] If an existing installation for the passage of two phase ormultiphase fluid has an element capable of providing mixing, such as ablind T installation, then the invention could be provided by theinclusion of two flow elements, each having means to measure thedifferential pressure of the fluid passing through that element,included downstream of the mixing element.

[0035] With this unique arrangement it has been demonstrated that highlyaccurate and consistent measurements of both gas and liquid can beobtained at various gas void fractions from 91% to 99.5% but could besuitable for a wider range of gas fractions.

1. A meter for the measurement of the gas mass flow rate and totalliquid mass flow rate of a two phase or multiphase fluid, the metercomprising two flow elements arranged to be positioned downstream of anelement arranged to mix or homogenise fluids passing therethrough, thetwo flow elements each having means to provide an indication of thedifferential pressure of the fluid passing through that element.
 2. Ameter according to claim 1, comprising a first flow element arranged tomix or homogenise fluids passing therethrough with the two differentialpressure measuring flow elements arranged, in use, downstream of thefirst flow element.
 3. A meter according to claim 1 or claim 2 whereinthe differential pressure measuring flow elements each present adifferent configuration to the fluids flowing therethrough.
 4. A meteraccording to claim 3, wherein independent differential pressuremeasurements are provided by each of the differential pressure measuringflow elements.
 5. A meter according to claim 3 or claim 4, wherein eachof the differential pressure measuring flow elements is arranged tocreate a pressure drop to fluid passing therethrough.
 6. A meteraccording to any of the preceding claims, wherein at least one of theflow elements comprises a conduit having one or more internalprojections therein.
 7. A meter according to claim 6, wherein theconduit has two projections which project inwardly from substantiallyopposite inner surfaces of the conduit.
 8. A meter according to claim 7,wherein the projections are staggered along an axial length of theconduit.
 9. A meter according to any of claims 6 to 8, wherein the firstmixing or homogonising flow element and one of the differential pressuremeasuring flow elements each comprise a conduit having one or moreinternal projections therein and the other differential pressuremeasuring flow element comprises a venturi.
 10. A meter according to anyof the preceding claims including a processing element which is arrangedto receive signals indicative of differential pressure measurements ofthe fluid passing through each of the differential pressure measuringflow elements and to determine the gas mass flow rate and total liquidmass flow rate of the fluid based on the received signals.
 11. A meteraccording to claim 10, wherein a pressure sensor is provided to measurethe upstream pressure of fluid passing through the meter and theprocessing element is arranged to receive a signal indicative of apressure measurement from the pressure sensor to be used in determiningthe gas mass flow rate and total liquid mass flow rate of the fluid. 12.A meter according to claim 10 or claim 11, wherein a temperature sensoris provided to measure the temperature of fluid passing through themeter and the processing element is arranged to receive a signalindicative of a temperature measurement from the temperature sensor tobe used in determining the gas mass flow rate and total liquid mass flowrate of the fluid.
 13. A meter according to any of claims 10 to 12,wherein the processing element is arranged to determine the gas massflow rate and total liquid mass flow rate of the fluid using algorithms.14. A meter according to any of claims 10 to 12, wherein the processingelement is arranged to determine the gas mass flow rate and total liquidmass flow rate of the fluid using a look-up table.
 15. A metersubstantially as hereinbefore described with reference to theaccompanying drawings.
 16. A flow element for use in a meter for themeasurement of the flow rate of two phase or multiphase fluid, the flowelement comprising a conduit having a fluid flow path and one or moreinternal projections projecting into the fluid flow path.
 17. A flowelement according to claim 16, wherein the flow element has twoprojections which project inwardly from substantially opposite innersurfaces of the flow element.
 18. A flow element according to claim 17,wherein the projections are staggered along an axial length of theconduit.
 19. A flow element substantially as hereinbefore described withreference to the accompanying drawings.
 20. A method of measuring thegas mass flow rate and total liquid mass flow rate of a two phase ormultiphase fluid, the method comprising mixing or homogonising the twophase or multiphase fluid by passing it through a first flow element,passing the mixed or homogonised fluid through a second flow element andmeasuring the differential pressure across the second flow element,passing the fluid through a third flow element and measuring thedifferential pressure across the third flow element and using thedifferential pressures measured across the second and third flowelements to determine either or both of the gas mass flow rate and theliquid mass flow rate of the passing fluid.
 21. A method according toclaim 20, wherein the second and third flow elements each present adifferent configuration to the fluids flowing therethrough.
 22. A methodaccording to claim 21, wherein independent differential measurements areprovided by each of the differential pressure measuring flow elements.23. A method according to any of claims 20 to 22, wherein each of thesecond and third flow elements is arranged to create a pressure drop tofluid passing therethrough.
 24. A method of measuring the gas mass flowrate and total liquid mass flow rate of a two phase or multiphase fluidsubstantially as hereinbefore described with reference to theaccompanying drawings.